Measurement of film thickness of integrated circuits

ABSTRACT

A narrow, high energy, electron beam is caused to impinge upon an integrated circuit. The accelerating voltage of the electron beam is increased in incremental steps (3 or more) so that the electrons penetrate into the film and then into the substrate. The transmitted electrons interact with the film and substrate materials to generate distinct X-rays. The relative X-ray intensities of the film material to that of the substrate material is utilized to determine the film thickness.

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto me of any royalties thereon.

TECHNICAL FIELD

The present invention relates to a method of measuring the filmthickness of integrated circuits using an energy dispersive X-rayanalysis (EDXA) technique.

BACKGROUND OF THE INVENTION

During the course of failure analysis or quality inspection andevaluation of microelectronic parts (ICs), a distinct need to measuremetallization or oxide thickness is required to evaluate the processingof chips and to perform failure analysis. A well known, prior arttechnique proposed for this purpose makes use of a Scanning ElectronMicroscope (SEM) in conjunction with an Energy Dispersive X-ray Analysis(EDXA) technique. This latter technique, commonly known as theYakowitz-Newbury method, has unfortunately proved too time consuming inpractice, it requires high accelerating voltage and consistentlyestimates the thickness of films at a higher value than the actualvalue. The inaccuracy of the Yakowitz-Newbury method is, in part,attributed to the relatively large thickness of the integrated circuitfilms, e.g., approximately 1 micron for aluminum films and 0.5 to 0.8microns for silicon diode/silicon nitride. Moreover, the highaccelerating voltage required by this prior art technique often resultsin damage to the integrated circuit(s).

SUMMARY OF THE INVENTION

It is the primary object of the present invention to achieve a method ortechnique for measuring the film thickness of integrated circuits whichis simple to implement, uses relatively low accelerating voltages, andyet achieves good measurement accuracy.

A related object is to obtain an accurate measurement of integratedcircuit film thicknesses in a relatively fast, yet nondestructive,manner.

The above and other objects are achieved in accordance with the presentinvention wherein an accelerating electron beam voltage is increased inincremental steps (3 or more) so as to detect the penetration of theelectron beam through an integrated circuit thin film. Once the electronbeam is of sufficient energy to penetrate the film, the transmittedelectrons interact with the substrate material to generate X-rays. Therelative X-ray intensities of the film material to that of the substratematerial can be used (e.g., plotted) to obtain a close estimate of theaccelerating voltage required to penetrate the film. This thresholdvalue for film penetration voltage is then used, in conjunction with arange-energy formula, to closely estimate the electron range and itscorrelate to film thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully appreciated from the following detaileddescription when the same is considered in connection with theaccompanying drawing, in which:

FIG. 1 is a schematic block diagram of the apparatus arrangement used incarrying out the technique of the present invention;

FIG. 2 is an enlarged showing of a portion of an integrated circuit andsuccessive electron beam penetrations therethrough;

FIG. 3 is plot using Yakowitz-Newbury calculations which allow for thedetermination of penetration energy;

FIG. 4 is a predicted or theoretical plot of relative X-ray intensities(Isub/If) vs. beam energy (E) in accordance with the technique of thepresent invention;

FIG. 5 is the actual or experimental data obtained for an Isub/If vs.beam energy plot; and

FIGS. 6-7 are waveforms showing typical X-ray counts obtained when anaccelerating electron beam impinges upon and penetrates an aluminum thinfilm mounted on a silicon substrate.

DETAILED DESCRIPTION

Turning now to FIG. 1 of the drawings, the integrated circuit 11comprises a film 12 of Al or Si0₂, for example, which is etched orprocessed to a particular design specification. It is the purpose of thepresent invention to measure the thickness of this film. To this end, anarrow, high energy, electron beam 13 is generated by a scanningelectron microscope (SEM) 14 and impinges upon the integrated circuit(IC). As is known to those in the art, the electron beam can be directedto a very specific point on the chip (+/- on micron). As theaccelerating voltage of the electron beam 13 is increased in intensitythe electrons will penetrate further into the chip. As a consequence,the transmitted electrons interact with the material(s) of the chip togenerate X-rays. These X-rays are then detected by the X-ray detector15, which is a commercially available item. The X-rays from the film andfrom the substrate are distinct since they are of different wavelengths(λ).

FIG. 2 illustrates an integrated circuit chip and impinging electronbeams of differing energies E1, E2 and E3, where E1<E2<E3. The threebeams would, of course, be directed to the same point or spot on thechip, but for illustrative purposes they are shown impinging upon threedifferent points. The electron beam E1 is of relatively low energy sothat it penetrates the film to a small extent. As the variableaccelerating voltage is increased the electron beam represented as E2penetrates the film to its interface with the substrate material. With afurther increase in the accelerating voltage, the electron beamrepresented as E3 will penetrate into the substrate. The arrows labeledIf and Isub represent the X-ray intensities which result from theinteraction of the electrons with the film and substrate materials,respectively. As will be understood by those in the art, the further theelectron beam penetrates into a material (e.g., the substrate) thegreater the intensity of the generated X-rays.

Before turning to the theoretical basis for the method of the presentinvention, and at the possible risk of some redundancy, it may perhapsbe advantageous to summarize at this point the significant aspects ofthis method invention. Basically, this method or technique uses avariable accelerating voltage to detect the penetration of the electronbeam through a thin film (i.e., an etched or processed integratedcircuit) using an energy dispersive X-ray analysis (EDXA) technique.Once the electrons have sufficient energy to penetrate the film, thetransmitted electrons can interact with the substrate material togenerate X-rays. By plotting the relative X-ray intensities of the filmmaterial to the substrate material, a close estimate of the acceleratingvoltage required for film penetration can be made. This threshold valuefor the film penetration voltage can be used in conjunction with arange-energy formula to estimate electron range and corollate to thefilm thickness.

Implementation of this method requires localization of the electron beamon the area of interest. This can be done with high accuracy andresolution, in contrast to optical methods. A minimum of three (andpreferably more) X-ray intensity readings beyond penetration is requiredto give a fair estimate of the penetration voltage. The greater thenumber of incremental increases in the accelerating voltage, the greaterthe accuracy in film thickness measurement.

It has been found that reasonable statistics can be obtained for allthree readings within a period of minutes. A simple calculation is thenemployed to obtain the film thickness.

The implementation of this technique is relatively insensitive to theelectron microscope operating conditions and can tolerate variations inthe electron beam parameters, provided the accelerating voltage isstable and accurate during measurement and data accumulation.

To examine the theoretical basis of the present penetration voltagemethod and to determine the appropriate formalism for implementing thismethod, two empirical/theoretical based formalisms are investigated,namely, Yakowitz-Newbury (Yakowitz-Newbury, 1976) and the Everhart-Hoff(Everhart-Hoff, 1971) formulations.

In the Yakowitz-Newbury formulation, k defines the ratio of the X-rayintensity from the film (If) to the X-ray intensity of the bulk sampleof the same material as the film (Ibf):

    K=If/Ibf                                                   (1)

Further, taking the case of the electron beam penetrating the film, thenit follows that If<Ibf. In this case, to a first approximation, theX-ray intensity generated in the substrate material (Isub) can beexpressed as:

    Isub=C(Ibf-If),                                            (2)

since any part of the electron beam not dissipated in the film will bedissipated in the substrate. C is an arbitrary constant to account forother effects, such as those arising at the interface. Dividing by If,the following useful expression is obtained: ##EQU1## This expression(Eq.3) will allow the determination of the penetration energy byplotting (1/k-1) vs. the electron beam energy. For the calculations, itwas assumed that a 1 micron thick aluminum film was deposited on asilicon substrate. The k factor was calculated for various energies and(1/k-1) plotted, as shown in FIG. 3.

Referring to FIG. 3, the penetration energy obtained at the intersectionof the line with the energy axis is approximately 12.3 keV. Usingvarious range-energy formulas, such as depth-dose (Everhart-Hoff, 1971)or the Heinrich formula (Yakowitz-Newbury, 1976), the respectivethicknesses obtained for the penetration voltage are 1.2 microns and1.58 microns. Clearly, to obtain the correct value for the filmthickness, a correction factor of 0.83 or 0.63 respectively would berequired. Further, actual experimental measurements on aluminum filmsgave similar results, where the Yakowitz-Newbury method estimated thefilm thickness much higher than the actual value of the film thickness.

To improve on the high estimation bias obtained with the above method,another thickness estimation method was employed, based on theEverhart-Hoff formulation. Using the depth-dose formalisms, the beamenergy dissipated in the material is given by: ##EQU2## where Z is thenormalize energy loss parameter, and y is the normalize depth. Using thesimplified assumption that the energy of the electron beam, notdissipated in the film, will be dissipated in the subtrate, thefollowing expression is obtained for the X-ray intensity ratio: ##EQU3##and yf is the normalized thickness of the film. Using this expression(Eq.5) a plot of Isub/If vs. electron beam energy can be obtained.Assuming as before, a 1 micron thick aluminum film on a siliconsubstrate, the plot of Isub/If is shown in FIG. 4.

The penetration energy obtained from the plot of FIG. 4 is approximately11.5 keV. The Everhart-Hoff formalism, in combination with thedepth-dose range-energy relation, gives the best results for estimatingthe thickness of aluminum film on integrated circuits. The abovediscussed formalisms confirm the correctness of the experimental methodas an estimation technique for the penetration voltage. The depth-doserelation was used to estimate the film thickness from the penetrationvoltage. This concludes the theoretical discussion of the basis for thevoltage penetration method of the present invention and, in turn,establishes the basis for the approach set forth below on theexperimental procedures.

To calibrate this method with respect to a secondary length standard, analuminum film of approximately 1 micron was deposited on two siliconwafers. One wafer was coated with a thin film of gold and the other wasleft uncoated. These wafers were fractured and the thickness of thealuminum film was measured visually by using an SEM. The calibratedmicron marker of SEM was compared to a secondary length standard(diffraction grading). This secondary standard was, in turn, calibratedwith respect to the NBS SRM 484. Based on this procedure, the SEMmeasurement of the film thickness is the most accurate of all themethods described herein, since it can be traced to a "primary" lengthstandard from the NBS. This visual SEM measurement technique is, ofcourse, a destructive one.

The EDXA technique of the present invention was used to measure the filmthickness in the vicinity of the SEM thickness measurements. Asdescribed previously, the accelerating voltage of the SEM was varieduntil the e-beam penetrated the aluminum film and a weak silicon peakwas observed. At this point, the voltage was increased in steps of 1 keVand the X-ray spectra was taken at each step. The relative ratio ofsilicon to aluminum (Isi/Ial) was then plotted vs. accelerating voltage(see FIG. 5). Enough data was taken to extrapolate back to thepenetration voltage. Usually, this required a minimum of three to fourdata points beyond the initial detection of substrate material. Further,enough counts were accumulated at each point to give a peak count forthe film above 30 k. Typically, the plot of the data is as shown in FIG.5. A least squares fit of the data points, above the minimum point, to astraight line, will allow an estimation of the penetration voltage atthe intersection with the energy axis.

                  TABLE I                                                         ______________________________________                                        Estimated Penetration Voltage (keV)                                           Al Film        Al Film  IC#1       IC#2                                       (uncoated)     (coated) (uncoated) (uncoated)                                 ______________________________________                                        Peak Ratio                                                                            10.21      11.44    12.89    13.16                                    (Isi/Ial)                                                                     K Ratio 10.52      11.25    --       --                                       (Ksi/Kal)                                                                     ______________________________________                                         (Uncertainty of +/- 0.2 keV)                                             

In the table the first column represents a thin film or wafer ofaluminum which is unetched; the second column represents a thin wafer ofaluminum also unetched, but gold coated; the columns labeled IC#1 andIC#2 represent two integrated circuit chip samples made of aluminum filmwith a silicon substrate. The latter samples are production sampleswhich are etched or fabricated IC chips. The K ratio was obtained forvalues produced after performing a semiquantitative analysis on eachspectra. The quickest way for implementing above procedure was by usingpeak ratio (Isi/Ial), since it didn't require further analysis orcomparison to standards. Also, the peak ratio was relatively insensitiveto SEM operating parameters as compared to other ratios.

When the estimated penetration voltage of the film is obtained, a closeestimation of its thickness can be obtained using an availablerange-energy formula. The depth-dose formula (Everhart-Hoff, 1971) wasused here:

    T=40×E.sup.1.75 /p                                   (7)

where T equals thickness of film (microns), E equals electron-beamaccelerating voltage (keV) and P equals density (mg/cm³). Using thisequation to determine film thickness, the results in Table II areobtained and these are compared with the SEM measurements The columnsare as previously described. While the estimated penetration voltage (E)might be obtained from a plot such as shown in FIG. 5, in practice theraw data would be delivered to a computer which would readilyextrapolate from the same to obtain a close estimation of thepenetration voltage (E).

                                      TABLE II                                    __________________________________________________________________________    Film Thickness Measurements(Microns)                                                 Al Film Al Film                                                                              IC#1    IC#2                                                   (uncoated)                                                                            (coated)                                                                             (uncoated)                                                                            (uncoated)                                      __________________________________________________________________________    Peak Ratio                                                                           0.863+/-.04                                                                           1.053+/-.04                                                                          1.298+/-.05                                                                           1.346+/-.05                                     K Ratio                                                                              0.909+/-.04                                                                           1.023+/-.04                                                                          --      --                                              SEM(visual)                                                                           1.00+/-.034                                                                          1.170+/-.04                                                                           1.55+/-.13                                                                           1.479+/-.12                                     __________________________________________________________________________

These results using the penetration voltage method of the invention arereasonable and give a credible estimate for the thickness of aluminumfilms. In contrast, use of the Yakowitz-Newbury technique consistentlygave film thickness estimates that were larger in value than the actualmeasured values, for the range of thicknesses and SEM operatingparameters used for this experiment. Further, the Yakowitz-Newburymethod requires much higher accelerating voltages to obtain thicknessestimates than the present penetration voltage method. The higheraccelerating voltages required by Yakowitz-Newbury can damage theintegrated circuit, causing the device to malfunction electrically. Withthe present penetration voltage method, there is far less likelihood ofdamage and, at worst, may only lead to degradation of parameters ratherthan complete destruction or malfunction of the integrated circuit.Also, damage or degradation can be further limited for sensitivecircuits by restricting the probing and measurements to the peripheralarea of the chip where none of the critically active circuit elementsare located. Based on the results obtained, the penetration voltagemethod gives a reasonable and fairly accurate measurement of the filmthickness of an integrated circuit with a minimum of possible damage tothe electrical functionality of the device.

To obtain even more precise estimates of the film thickness, acorrection factor was applied to give a one micron thickness for theuncoated, peak ratio reading. This would correspond with the SEM visualanalysis of that film. Applying the same correction to all the othersamples, the results in table 3 are obtained.

                  TABLE III                                                       ______________________________________                                        Corrected Thickness Estimates(Microns)                                               Al Film Al Film   IC#1      IC#2                                              (uncoated)                                                                            (coated)  (uncoated)                                                                              (uncoated)                                 ______________________________________                                        Peak Ratio                                                                             1.00      1.22      1.50    1.56                                     K Ratio  1.05      1.18      --      --                                       SEM(visual)                                                                            1.0       1.17      1.55     1.497                                   ______________________________________                                    

These results correspond very closely with the SEM analysis values,which can be traced to secondary and NBS length standards. Based on theclose correlation of the corrected penetration voltage thicknessestimates with the SEM visual, the correction factor is valid for allfour samples, taken from different sources. Based on these results, thecorrection factor can be utilized for other aluminum film samples tomore closely estimate the film thickness.

Similar results were obtained for silicon dioxide/silicon nitride films,such as, passivation on integrated circuits. Typical results for ahybrid integrated circuit chip are given in table IV.

                  TABLE IV                                                        ______________________________________                                        Silicon Dioxide/Nitride Film Thickness                                                    Penetration                                                                   Voltage Thickness                                                             (keV)   (MICRON)                                                  ______________________________________                                        Peak Ratio    4.41      0.2429+/-.03                                          K Ratio       4.64      0.2655+/-.03                                          SEM(visual)   --        0.1980+/-.03                                          ______________________________________                                    

The results were obtained for a film of silicon dioxide. For a siliconnitride film, the penetration voltage thickness results would, mostprobably, have been closer to the SEM visual results and well within theerror margin. These results are quite reasonable and clearly demonstratethe utility of the penetration voltage method and its applicability tosilicon dioxide/nitride films. FIGS. 6-7 are plots of X-ray counts(ordinate) versus X-ray energy (abscissa) for an accelerating electronbeam impinging upon an aluminum (atomic No. 13) film mounted upon asilicon substrate. The vertical scale only extends to a count of 4096.The horizontal scale extends from 0.210 keV to 2.770 keV, with thecursor (K α) at 1.490 keV. An electron beam of 16 keV produces an (Al)X-ray count of 26,913 (FIG. 6). The X-ray count for the siliconsubstrate is negligibly small by comparison. This corresponds to the E2situation indicated in FIG. 2. When the accelerating voltage isincreased to 18 keV (FIG. 7), the electron beam penetrates into thesubstrate (E3, in FIG. 2) and the X-ray intensity from the substrate(Isub) increases significantly. The value of If, of course, alsoincreases. The designation 30A indicates a 30° take-off angle.

The utility and accuracy of the penetration voltage method of thepresent invention as a means of obtaining the thickness of aluminum andsilicon dioxide/nitride thin films on integrated circuits, in the 0.5 to1.5 micron range, has been demonstrated. Further, it will be evident tothose skilled in this art that this penetration voltage method has clearapplicability to the measurement of thick films as well. It isanticipated that this method will prove to be a useful technique whenused with MIL-STD 883, Method 2018.2.

Having thus shown and described what is at present considered to be thepreferred method, it should be understood that the same has been shownby way of illustration and not limitation. Accordingly, allmodifications, alterations and changes coming within the spirit andscope of the invention as defined in the appended claims are hereinmeant to be included.

What is claimed is:
 1. A method of measuring the film thickness ofintegrated circuits comprising the steps of:directing a narrow, highenergy, electron beam to impinge upon an integrated circuit; increasingin incremental steps of three or more the accelerating voltage of theelectron beam so that the electrons penetrate into the film and with oneor more further increases penetrate into the substrate of the integratedcircuit to a predetermined extent; the transmitted electrons serving tointeract with the film and substrate materials to generate distinctX-rays; detecting the generated X-rays; the relative X-ray intensitiesof the film material to that of the substrate material being used toobtain a close estimate of the accelerating voltage required topenetrate the film; determining the film penetration voltage inaccordance with a least squares fitting of the relative X-ray intensitydata points or readings; and calculating the film thickness (T) inaccordance with the formula:

    T=40×E.sup.1.75 /P

where E=film penetration voltage, and p=film material density in mg/cm³.